This invention relates generally to the control of uninterruptible power supplies, and more particularly to the control and coordination of multiple power modules in a modular uninterruptible power supply.
As more and more segments of the business environment enter the information age, more and more computers and computing power are required. As business move from the old to the new economy their reliance on the processing, transference, and storage of digital information is becoming a more and more critical aspect of their overall business strategy. While in the past, computer crashes were seen as a mere nuisance, the loss of computing power and business data may well devastate a business""s ability to survive in today""s new economy. As such, the need for reliable, uninterruptible electric power to maintain the operational status of the computing equipment and the integrity of the digital data continues to rise.
To meet these requirements, uninterruptible power supplies (UPS) have been developed. These UPSs utilize a bank of electric storage batteries and solid state conversion equipment in association with the utility line voltage to provide continuous electric power to a business""s computer system in the event of a loss or deviation of power quality from the utility. The number of batteries contained within an UPS is dependent upon the business""s length of time that it needs to operate in the event of a utility power system failure. Likewise, the number of power modules included in a modular UPS is dependent on the overall total system load required to be supplied thereby. In the past, uninterruptible power supplies were only available in discreet sizes. However, with the recognition of the various requirements from the multitude of businesses that require UPSs, the development of modular uninterruptible power supplies has provided a flexibility heretofore unknown. Now, a business""s UPS power output may grow as its business needs by simply purchasing additional power modules and adding them to their existing modular UPS.
Unfortunately, the advent of the modular UPS has also introduced new and different challenges to the UPS designer. Specifically, the modular UPS must function as a single system regardless of the number of power modules that are added to or taken away from the UPS chassis. Output power quality, system display, and transitions between operating modes must all be coordinated between the multiple power modules that make up the business""s UPS system.
One such modular UPS that provides coordinated operation is described in U.S. Pat. No. 5,982,652 to Simonelli, et al. for a METHOD AND APPARATUS FOR PROVIDING UNINTERRUPTIBLE POWER USING A POWER CONTROLLER AND A REDUNDANT POWER CONTROLLER. As described in this patent, the coordination and control of the multiple power modules contained in the modular UPS is accomplished through the provision of a main intelligence module (MIM) and a redundant intelligence module (RIM). The MIM and RIM have connections to each other and to the other modules to allow monitoring of operational conditions of the UPS system, including characteristics of the input and output power, and to provide phase synchronization, frequency regulation, and voltage regulation of the output power. In this system, the MIM functions as the primary controller within the UPS system and the RIM is a redundant controller that can assume control of the UPS system upon failure of the MIM or removal of the MIM from the UPS system. All of the power modules are substantially identical and perform the functions of an uninterruptible power supply under the control of the MIM or the RIM.
In the system of the Simonelli, et al. ""652 reference, the main intelligence module (MIM) is the primary computer/controller in the UPS system. It acts as a central point in the system for collecting and communicating information about aspects of the power modules, battery modules, and the redundant intelligence module (RIM). The MIM contains a primary and a backup microprocessor for controlling and coordinating this operation. The redundant intelligence module (RIM) on the other hand is merely a backup version of the MIM, and provides redundancy in the event of a MIM failure, or while a MIM is being replaced. In this backup role, the RIM includes the same major systems as the MIM except for the main processor system, although some of the systems in the RIM are described as having less functionality than the corresponding systems in the MIM.
Unfortunately, such a control strategy greatly increase the overall cost of the modular UPS system by requiring that two separate additional modules be purchased and maintained in order to allow functionality of the UPS at all. In this way, in addition to the value providing power modules, battery modules, and battery charging modules, a user is forced to also purchase a separate main intelligence modules to control the other components in the system. Further, the user is forced to purchase a redundant intelligence module (RIM) whose only main purpose is to provide backup operation should the MIM fail. In other words, this module is totally superfluous to the actual operation and functionality of the uninterruptible power supply unless a failure of the MIM occurs.
In an economy that demands maximum efficiency from business resources, such a system that requires separate primary and redundant control modules simply cannot be justified. The unfeasibility of such a system may be clearly demonstrated in a situation where a business enterprise only needs a single power module to supply its uninterruptible power requirements. In such a situation, the Simonelli, et al., ""652 system would require the business enterprise to purchase two control units, the MIM and the RIM, to control the single power module in its UPS. In such a situation it becomes clear that the provision of two control units to control a single power module is a luxury not many businesses would desire to acquire.
Therefore, there exists a need in the art for a modular uninterruptible power supply that has coordinated control of its constituent components and an acceptable level of redundancy without requiring the purchase of additional multiple control unit.
An embodiment of the invention provides a method and apparatus for coordinating the control of uninterruptible power supply modules to provide scalable, redundant power. Each of the independent UPS power modules are designed to contain identical control logic, both hardware and software, so that only a single power module need be purchased to scale the UPS system based on the business enterprises power requirements. Each of these identical UPS power modules are connected to common communications, power, and logic buses within the UPS system. Under such a system, the multiple power modules act as a single UPS with all of the controls and actions taken by the individual power modules being coordinated through a single power module acting as the system""s virtual master. Preferably, the master role is virtual because any of the power modules in the system contain all of the necessary control logic and software to assume the master role.
For the system of the invention to be fault tolerant it must be capable of withstanding a failure or removal of the power module that has assumed the master role. Preferably, this is accomplished by requiring another module to be prepared to provide backup to the master. In this role, designated vice-master, the power module provides the controlling signals to itself and the other power modules if and when the designated master fails. Under this system, the vice-master then promotes itself to master and assigns a new vice-master from one of the operational power modules in the system. Under the system of the invention, no additional main intelligence modules or redundant intelligence modules are required to perform this functionality. Coordination and redundancy are provided within the control of the individual power modules themselves.
A power up arbitration scheme is used to assign one of the power modules to the master""s role. Preferably, the master then assigns a vice-master and assigns all other modules to act as peers. Peer modules simply follow the control commands of the master or vice-master depending on the operational status of these two units. Modules in the peer role are also drawn upon when a vice-master needs to be appointed to the master role upon detection of a problem with that unit. This hierarchy of roles provides the means to coordinate identical UPS modules into a larger UPS system without the need for additional special main and backup controllers. In the event that only a single power module is connected in the UPS system, it is quite capable of controlling itself in the role of the master, without a requirement for a vice-master or any peer modules to be connected. Under the system of the invention, there is no need for two separate control modules to control the one power module connected in the system.
Power module interconnection and coordination of control is accomplished using a common set of signals, collectively known as the redundancy management bus (RMB). The RMB of the invention includes a high-speed communication bus, a set of bi-directional logic lines, and one analog reference for each power phase of the UPS system. Each power module may drive any of the signals. It is the role of the power module that determines which signals are driven and which signals are merely monitored. The high-speed communication bus may take any one of a number of forms, including CAN, Ethernet, Serial 485, etc. In a preferred embodiment, a high-speed communication bus takes the form of a CAN bus due to CAN""s inherit fault tolerance and noise immunity.
In accordance with the invention, the master is responsible for driving most of the signals on the RMB. The other power modules take direction from the master by acting on these signals. When the master power module fails or is removed from the UPS system, a master ok signal typically supplied by the master power module is no longer sent. This provides an indication that the vice-master power module should take over driving the analog output voltage target signal used by the power modules to create their output voltage waveforms. Since the role of the vice-master has previously been assigned, the remaining peers do not see a discontinuity in the output voltage target signal and therefore their inverter output waveforms are clean during this transition.
In a preferred embodiment of the invention, the microprocessor inside each of the power modules is connected to the RMB through an interface computer programmable logic device (CPLD). Preferably two digital signals from the processor are used to indicate the power module""s role to the CPLD. The role logic is used to enable bus drivers for each role""s group of output signals. The power module""s particular role in a given UPS system determines the bus input/output direction for each communicated signal.
In a preferred embodiment of the invention, the microprocessor inside each of the power modules is connected to the RMB through an interface computer programmable logic device (CPLD). Preferably two digital signals from the processor are used to indicate the power module""s role to the CPLD. The role logic is used to enable bus drivers for each role""s group of output signals. The power module""s particular role in a given UPS system determines the bus input/output direction for each communicated signal.